This invention relates to a lithium ion secondary battery comprising a positive electrode and a negative electrode facing each other via a separator supporting an electrolyte. More particularly, it relates to a thin type lithium ion secondary battery having excellent charge and discharge characteristics.
There has been an eager demand for reduction in size and weight of portable electronic equipment. To meet the demand, it is required not only to improve battery performance, such as high voltage, high energy density, and resistance to high load, but to widen the freedom of shape design and to secure safety. Development and improvement of a variety of batteries have been proceeding along these lines. Of currently available batteries, lithium ion batteries are the most promising secondary batteries for realizing a high voltage, a high energy density, and resistance to high load and the most expected to fulfill the above-described requirements.
A lithium ion secondary battery mainly comprises a positive electrode, a negative electrode, and an ion conducting layer interposed between the electrodes. The lithium ion secondary batteries that have been put to practical use employ a positive plate prepared by mixing a powdered active material, such as a lithium-cobalt complex oxide, and a powdered electron conductor with a binder resin and applying the mixture to an aluminum current collector; a negative plate prepared by mixing a powdered carbonaceous active material with a binder resin and applying the mixture to a copper current collector; and an ion conducting layer made of a porous film of polyethylene, polypropylene, etc. filled with a nonaqueous solvent containing lithium ions.
In the lithium ion secondary batteries that have been put to practical use, electrical connections among the positive electrode, the ion conducting layer, and the negative electrode are maintained by pressure application by using a firm battery case made of stainless steel, etc. However, such a case increases the weight of a lithium ion secondary battery to make it difficult to realize size and weight reduction. Moreover, the rigidness of the case narrows the freedom of shape design.
In order to achieve size and weight reduction and freedom of shape design of a lithium ion secondary battery, it is necessary to bond an ion conducting layer to a positive electrode and to a negative electrode and to maintain the joined state without applying pressure from the outside.
In this connection, U.S. Pat. No. 5,437,692 discloses a structure in which a lithium ion-conducting polymer is used as an ion conducting layer, and a positive electrode and a negative electrode are joined to the ion-conducting layer with an adhesive layer containing a lithium compound. WO95/15589 discloses a structure having a plastic ion-conducting layer to which a positive and a negative electrode are joined.
According to the method taught in U.S. Pat. No. 5,437,692 supra, however, the joint strength attained is not enough, the battery cannot be made sufficiently thin, and the ion conduction resistance between the positive and the negative electrodes through the ion-conducting layer is high so that the battery characteristics such as charge and discharge characteristics are insufficient for practical use. According to WO95/15589 supra, the ion-conducting layer, being plastic, cannot secure sufficient joint strength, and the thickness of the battery cannot be reduced sufficiently.
The present invention has been made in order to solve these problems. It provides a battery structure in which a positive and a negative electrode are brought into intimate contact with an ion-conducting layer (hereinafter referred to as a separator) with an adhesive resin to secure sufficient joint strength among the electrodes and the separator while suppressing ion conduction resistance among them on the same level as in a conventional battery put in a case.
A first lithium ion secondary battery according to the present invention comprises a plurality of laminates each having a separator holding an electrolytic solution to which a positive electrode and a negative electrode are joined with an adhesive resin layer having a mixed phase composed of an electrolytic solution phase, a polymer gel phase containing an electrolytic solution, and a polymer solid phase.
A second lithium ion secondary battery of the invention is the above-described 1st battery, wherein the plurality of laminates are formed by interposing the positive electrode and the negative electrode alternately among a plurality of cut sheets of the separator.
A third lithium ion secondary battery of the invention is the above-described 1st battery, wherein the plurality of laminates are formed by interposing the positive electrode and the negative electrode alternately between rolled separators.
A fourth lithium ion secondary battery of the invention is the above-described 1st battery, wherein the plurality of laminates are formed by interposing the positive electrode and the negative electrode alternately between folded separators.
A fifth lithium ion secondary battery of the invention is the above-described first battery, wherein the polymer gel phase and the polymer solid phase contain the same kind or different kinds of polymeric materials, and the polymeric material contained in the polymer gel phase and that contained in the polymer solid phase have different average molecular weights.
A sixth lithium ion secondary battery of the invention is the above-described first battery, wherein the polymer gel phase and the polymer solid phase contain polyvinylidene fluoride, and the polyvinylidene fluoride contained in the polymer gel phase and that contained in the polymer solid phase are different in average molecular weight.
A seventh lithium ion secondary battery of the invention is the above-described first battery, wherein the polymer gel phase and the polymer solid phase contain polyvinyl alcohol, and the polyvinyl alcohol contained in the polymer gel phase and that contained in the polymer solid phase are different in average molecular weight.
In the first to seventh lithium ion secondary batteries, both joint strength and ion conductivity among the positive electrode, the negative electrode, and the separator can be obtained by adhesion with the above-mentioned adhesive resin layer which has a mixed phase made up of an electrolytic solution phase, a polymer gel phase containing an electrolytic solution, and a polymer solid phase. Owing to the adhesive strength and high ion conductivity thus secured, there is provided a practical compact lithium ion secondary battery of laminated electrode type which has a plurality of electrode laminates and yet requires no firm battery case and has high performance and a large capacity.
A process for producing the first lithium ion secondary battery according to the present invention is a process for producing a lithium ion secondary battery comprising a plurality of laminates each having a separator holding an electrolytic solution to which a positive electrode and a negative electrode are joined, which comprises coating the facing sides of a plurality of separators with an adhesive comprising two or more polymeric materials different in average molecular weight dissolved in a solvent, joining a positive electrode and a negative electrode alternately among the plurality of the separators via an adhesive resin layer to form a battery body, and impregnating the battery body with an electrolytic solution to make the adhesive resin layer a mixed phase composed of a polymer gel phase containing the electrolytic solution, a polymer solid phase, and an electrolytic solution phase. On being impregnated with an electrolytic solution, the adhesive resin layer becomes a mixed phase composed of a polymer gel phase containing the electrolytic solution, a polymer solid phase, and a layer of the electrolytic solution. Both the joint strength and ion conductivity between each of the positive and negative electrodes and the separator can be secured thereby. The thus laminated units can be piled to provide a practical and stable lithium ion secondary battery which can take a compact, thin, and arbitrary shape, having no firm battery case, and also exhibits high charge and discharge efficiency.